An extract from The Diana Chronicles, by Tina Brown, published in Vanity Fair, says Diana fell for the doctor while visiting a friend's husband in hospital.
"Isn't he drop dead gorgeous!" she said of the doctor after he left the room.
Diana then proceeded to visit the friend's surprised husband for 17 days straight in order to meet his doctor.
Brown writes that in no time, her Kensington Palace apartment "was fragrant with the scent of burning joss sticks".
She became a student of cardiology, got the anatomical textbook Gray's Anatomy and watched the TV show Casualty every Saturday night.
Once caught by a News of the World photographer at the hospital at midnight, she rang the paper's royal correspondent and planted a story that she went there three nights a week to comfort terminally ill patients.
Although Diana's life was been covered in excruciating detail, (there are 14 books coming out to mark the 10th anniversary of her death) Brown provides details of Diana's detachment from reality, such as telling Brown over a lunch that she thought she could solve the conflict in Northern Ireland.
"I'm very good at sorting out people's heads," Brown quotes her as saying. The New York Times reviewed the book, criticising Brown for relying on earlier books about Diana, but also saying: "At its best this book combines gossip, opinion and context in ways that cast new light on Diana's cautionary tale."
The extract in Vanity Fair, which Brown once edited, is focused on Diana's life after her divorce from Prince Charles.
For a while, the relationship between the Princess and the surgeon bloomed, she writes, but he was not interested in high society or fashion, didn't want anything from her, and had a dread of publicity.
Diana intended to turn a former equerry's room in Kensington Palace into a room for Natty, as he was known. On weekends, when her staff were not there, she would cook dinner for him.
"Marks and Spencer have got these very clever little meals that you just put the timer on and press the button and it's done for you!" Diana is quoted as telling her therapist, Simone Simmons.
On his birthday she went out to meet him wearing her best sapphire and diamond earrings, a fur coat and nothing else.
Her trips to Pakistan with Jemima and Imran Khan were covers to ingratiate herself with his family. She once turned up without notice to his family's house in Lahore, where she lay on the floor watching cartoons with the youngest children, but her efforts were in vain. Dr Khan's mother had no intention of letting her son marry anyone other than a Pakistani Muslim girl.
The episode deeply upset Dr Khan, who had concluded that Diana, abandoned by her mother when she was six, needed more love than any man could give her.
Diana's dream was that they would become international humanitarians. They would live somewhere sunny, with a pool - Australia or South Africa, Brown says.
Diana was delusional about the relationship. She talked to Jemima Khan late at night about how to handle marriage to a traditional Muslim man. She got her butler Paul Burrell to ask a priest if it was possible to marry secretly.
Brown writes: "The priest told him it was impossible to marry a couple without notifying the authorities - let alone without notifying the fiance, as it turned out."
Dr Khan was horrified when he found out. He had doubts about the relationship, and at the urgings of his family, met her at 10pm one night in Hyde Park in London and broke it off.
Diana went into a spiral after that. Increasingly isolated, she cut off her former sister-in-law Sarah Ferguson, and her mother, Frances Shand Kydd, who had become an alcoholic, and who had objected to her relationship with "a Pakistani and a Muslim".
She became dependent on Mr Burrell, and paranoid about the royal family. Brown says Diana told an Argentinian friend, Roberto Devorik, that Prince Philip wanted her to disappear.
"They are not going to kill me by poisoning me, or in a big plane. They will do it when I am in a small plane, in a car when I am driving, or in a helicopter."
The book is due to be published in the US next week.